A typical condensing gas furnace includes an induced draft blower that (a) pulls combustion air into the gas furnace, (b) pulls combustion gases (flue gases) resulting from igniting a mixture of the combustion air with gaseous fuel through a heat exchanger of the gas furnace, and (c) pushes the combustion gases out through venting ducts (vents) attached to the gas furnace. For practical reasons, the gas furnace is designed for use in different applications that may require different venting conditions, such as, but not limited to, direct venting, non-direct venting, short vents, long vents with elbows, etc. To ensure that the gas furnace functions under the different venting conditions, the induced draft blower of the gas furnace is generally operated at high RPMs. Operating the induced draft blower at high RPMs allows the combustion gases to be pushed out through the vents when the gas furnace is attached to long vents and/or vents with elbows. However, when the gas furnace is used with vents having shorter length and/or vents having minimal or no elbow, operating the induced draft blower at high RPMs reduces the efficiency of the gas furnace. The high RPM of the induced draft blower causes the combustion gases to flow through the heat exchanger of the gas furnace rapidly without having adequate time for efficient thermal transfer before being exhausted through the vents having shorter length with minimal or no elbow. That is, conventional gas furnaces are not adaptable to work under different venting conditions without compromising the efficiency of the gas furnaces.
Further, in conventional gas furnaces, to meet safety standards such as 1ANSI Z21.47, ANSI Z21.20, National Electric Code, CAN/CSA C22.2 No 199-M89, etc., electrical contacts of a pressure switch which confirms proper operation of the induced draft blower are typically connected in series with a relay controlling the gas valve. The series electrical connection between the pressure switch and the relay that controls the gas valve of the gas furnace allows the safety standard to be met by shutting off the gas valve output and ending the heating sequence in the event that the electrical contacts of the pressure switch are opened, even for a very short period of time. The electrical contacts of the pressure switch may be opened responsive to transients in pressure caused by conditions such as, but not limited to, (a) the impeller wheel of the induced draft blower passing over the pressure switch measuring port, (b) water temporarily blocking the pressure switch measuring port and (c) wind gusts blowing into the furnace exhaust vent. In conventional gas furnaces these conditions cannot be ignored due to the quick loss of flame once the electrical contacts of the pressure switch are opened and consequently the gas valve is de-energized. Every time the heating sequence of the gas furnace is ended, it takes several minutes to recover and re-start the heating sequence of the gas furnace which may be inconvenient and may negatively affect the efficiency of the gas furnace.
To prevent the shutting down and restarting of the heating sequence resulting from transients in pressure, in conventional gas furnaces, the induced draft blower is operated at a RPM considerably higher than that needed to close the electrical contacts of the pressure switch. Operating the induced draft blower at higher RPMs ensures that the heating sequence of the conventional gas furnaces does not shut off unnecessarily as a result of transients in pressure. However, as discussed above, operating the induced draft blower at higher RPMs results in reduced efficiency of the conventional gas furnaces.
In light of the above mentioned shortcomings of conventional gas furnaces, there is a need for a gas furnace with an improved control of the induced draft blower to maximize the efficiency of the gas furnace. It is noted that this background information is provided to reveal information believed by the applicant to be of possible relevance to the present disclosure. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present disclosure.